stolz

Hello, I have a vintage pair of Realistic PRO-50 headphones which I believe are the same as the AKG K250 600 ohm. (Made for Radio Shack by AKG.) They sound fine except for a very poor bass response. If I play them through either a software or hardware equalizer with the base controls pushed up high I do get a better bass response. I am using them with a Schiit Magni 3 amplifier set on the high position. I also get the same results with other headphone amps. Is this normal for this headphone? Are there any modifications to the headphones that I can make to improve the bass response? Many thanks in advance for any help.